Black pattern has been a characteristic problem for charge transfer sensors, and especially for CCD image sensors. Since a CCD operates by controlling a spatially defined array of depletion regions (potential wells), minority carriers continually generated due to thermal vibration of the silicon lattice tend to flow to the point of minimum potential where they are collected. Such thermally generated carriers are evidenced by what is known as black or dark current in the output of the sensor. In a conventional CCD sensor, a light protected area on the sensor (outside the image area) provides an estimate, called an offset, of the average dark current produced by the sensor. As shown in FIG. 1, this offset is subtracted from each image 1a, 1b, and 1c.
The dark current, however, is non-uniform throughout the sensor device, and imposes a fixed-pattern noise on the signal. This problem causes the image data from the CCD sensor to have a corresponding noise pattern. Importantly, the spatial pattern is stationary as to the device, although the amplitude at any particular location is dependent upon temperature, exposure time, and location in the array. Consequently, each pattern of amplitudes is unique to a device, as well to the temperature and the exposure time.
When linear CCD sensors require a black pattern correction, the correction is generally performed by storing a representative line of data captured in the dark. This line of data, which represents the residual black or dark current, is then subtracted from each line of data representing image information. The result of the subtraction is an image line free of the black pattern noise. This correction, however, assumes the pattern in the dark line remains the same as the dark current in the image line. Therefore, the dark line must be updated regularly to accommodate changes in temperature and exposure time.
Area CCD image sensor arrays often require a black pattern correction also. For area arrays, an array of lines is captured as a frame of data, and the dark current accordingly has a two-dimensional black pattern. To correct for such a black pattern, as shown in FIG. 2, it is known to capture a dark frame 2a representing an array of lines exposed in darkness, and then to subtract that dark frame from a contemporaneously-captured image frame 1a to obtain a corrected image frame 3a. Since the amount of dark current is a function of temperature and the length of exposure, the dark frame values must be updated regularly to accommodate changes in temperature and exposure time. This conventionally means, as shown in FIG. 2, that a unique dark frame 2a, 2b, or 2c is captured for each image frame 1a, 1b, or 1c. In addition, an average offset is still calculated from the light-protected area of the sensor for each dark and image frame, and subtracted as shown in FIG. 2 from the dark frames 2a, 2b, and 2c and the corrected image frames 3a, 3b, and 3c.
A problem arises because of the processing requirements for the dark frame updates necessary for black pattern correction. There are basically two known ways of handling these updates. For many imaging systems the exposure time changes regularly due to light variations. One solution, therefore, is to store dark frame values for all possible exposure times. However, since each frame can require megabytes of storage, such large amounts of storage are not always available. Moreover, the cost of memory sometimes precludes the dedication of so much memory to one correction.
The alternative is to acquire a new dark frame each time the exposure time changes. This methodology substantially affects system performance. Essentially every image frame acquisition requires an additional dark frame acquisition. This effectively doubles the amount of time required to perform a scan of the image sensor, which becomes especially prohibitive for long exposures.